This invention is related to an indicator particularly useful for indicating the temperature of a bus duct reaching an abnormal level. A bus duct is metal raceway the supports and contains a group of electrical bus bars. The duct comes in standard 10 foot lengths. Occasionally the bus bar joints become hot because of loosening of their compressed bolted connections which must be torqued and kept at a set foot pound value. Bolted joint connections must be re-torqued or re-tightened yearly, or as a bus bar manufacturer may specify. The problem is that it is not visually apparent that the bar temperature is rising to an abnormal level at the joint. Some electrical devices tend to give such an indication, but they are inadequate for a variety of reasons.
Some visual indicators of the prior art include U.S. Pat. No. 4,480,749 which was issued Nov. 6, 1984, to Peter K. Laucis and Robert Terry. This patent discloses a thermally sensitive indicator which uses a temperature sensitive liquid which changes color in response to a temperature of no less than 150.degree. . The indicator visually indicates whether an excessive amount of heat has been applied to the container.
U.S. Pat. No. 4,471,711 which was issued Sept. 18, 1984, to Dennis I. Graham, discloses a conduit which experiences a color change to indicate that the conduit has been exposed to excessive heat. Smoke, dust, dirt and oil film which are the results of a manufacturing process, make it difficult to detect a color change.
U.S. Pat. No. 3,586,320 which was issued Jun. 22, 1971, to John A. Mace, discloses a sign that drops due to a high flame applied to wax.
U.S. Pat. No. 1,595,734 which was issued Aug. 10, 1926, to William H. Scherer, discloses a thermal-release temperature indicator.
U.S. Pat. No. 2,983,247 which was issued May 9, 1961, to Seymour A. Greehspon discloses a high temperature tell-tale which releases a weight in response to a temperature increase.